1. Field of the Invention
The present invention relates to an aromatic polyimide laminate sheet and a process for producing the same. More particularly, the present invention relates to an aromatic polyimide laminate sheet having a desired relatively large thickness of 50 .mu.m or more, which thickness is difficult to obtain by a usual dope solution-spreading method, and consisting of one or more types of aromatic imide polymers alone having an excellent heat resistance, durability, flexibility and bonding property, and thus useful for various uses, and a process for producing the same easily and with an improved reproductivity.
2 Description of the Related Art
It is known that a conventional aromatic imide polymer film having a thickness of from about 10 to 130 .mu.m is produced by preparing a dope solution of an aromatic imide polymer or a precursory polymer of the imide polymer in an organic solvent, by spreading the dope solution on a substrate surface to form a thin layer of the dope solution, and by solidifying the dope solution layer to provide a thin sheet. However, the conventional dope solution-spreading method is disadvantageous in that the polymer has a poor solubility in an organic solvent, the resultant dope solution has a high viscosity, and therefore, the film-formation can be effected only by a difficult operation and under restricted conditions.
Accordingly, in the conventional method, it is very difficult to produce a sheet having a relatively large thickness of, for example, 50 .mu.m or more, with satisfactory quality and at high reproductivity.
Also, it is known that almost all aromatic imide polymers are non-fusible. Therefore, it is believed that a direct bonding of the conventional aromatic imide polymer films to each other without applying a bonding material, for example, a bonding agent or a bonding film, e.g., a fluorine-containing thermoplastic resin film, is very difficult.
Even if the aromatic imide polymer films can be bonded to each other with a bonding material, the resultant laminate sheet contains a bonding material layer which usually has a poorer heat resistance and durability than those of the aromatic imide polymer film, and thus exhibits, as a whole, a decreased heat resistance and durability.
Accordingly, there is a strong demand for the provision of an aromatic imide polymer sheet having a relatively large thickness of 50 .mu.m or more and free from a bonding material, and a process for producing the same at a satisfactory reproductivity and efficiency.